1. Field of the Invention
This invention relates to apparatus for detecting and interrupting currents in circuits experiencing arc faults. More particularly, it relates to arc fault detectors and circuit breakers incorporating them which respond to arc faults of a magnitude below the rated current for the circuit.
2. Background Information
Arc faults can occur in electrical systems for instance, between adjacent bared conductors, between exposed ends of broken conductors, at a faulty connection, and in other situations where conducting elements are in close proximity. Arc faults typically have high resistance so the arc current is below the trip thresholds of conventional circuit breakers. In particular, series arcs by their nature are limited by the load and, therefore, draw currents no greater than rated current for the circuit breaker. Even so, the arcs can cause damage or start a fire if they occur near combustible material.
Much attention has been directed toward trying to distinguish series and parallel arcing currents from other intermittent currents. It has been recognized that arcing currents generate a step increase in current when the arc is struck. However, many typical loads generate a similar step increase, such as for instance, when the device is turned on. In many instances the step increases generated by these loads are singular events while an arc fault generates a series of step increases. The arc fault detector described in U.S. Pat. No. 5,224,006 counts the step increases in current and generates a trip signal if a selected number of step increases occur within a given interval. However, there are loads which also generate repetitive step increases in current. This problem is addressed by the arc fault detector in U.S. Pat. No. 5,691,869 in which the arcing current is passed through a bandwidth limited filter which generates pulses having an amplitude proportional to the step increases. The arcing indication is generated when a time attenuated accumulation of these pulses reaches a predetermined value. Thus, a few very large magnitude step increases within a period of time, or a larger number of more modest step increases within a similar time period, generate a trip signal.
The above arc fault detectors primarily respond to step increases in current which exceed the rated current of the circuit breaker in which they are typically incorporated to avoid false trips in response to current signatures similar to arc faults produced by many household appliances. Presently, comprehensive arc fault detection requires complex signal processing of the current signatures requiring expensive electronic hardware. For instance, the arc fault detector described in U.S. Pat. No. 5,452,223 compares successive cycles of the arc current point by point and then applies a set of rules to the comparison to identify an arc fault. This requires expensive processing such as by a microcomputer which adds substantially to the cost of the detector.
There is a need, therefore, for an improved detector and circuit breaker incorporating the same for responding to arc faults.
In particular, there is a need for such an arc fault detector which responds to arc faults below as well as above the rated current of the circuit breaker and avoids false tripping in response to somewhat similar current signatures produced by some loads.
There is an important need for such an improved arc fault detector which can be produced economically.